1. Field of the Invention
This invention relates in general to a light sensing apparatus, and in particular to a light sensing apparatus employing a solid state light sensing MOS structure characterized by extremely low dark current (and corollarily long charge-holding capability, as well as the capability of sensing extremely low light levels) and spectral sensitivity that may correspond very precisely to the visible spectrum.
2. Description Relative to the Prior Art
Although solid state image sensing has generally been based on the use of silicon, its band gap of 1 eV is not optimal for low-level visible light sensing. The 1 eV band gap results in response well beyond the visible, and produces relatively high dark current levels.
Semiconductor devices comprised of gallium arsenide phosphide (GaAs.sub.1-x P.sub.x where x is the compositional parameter) have been discussed in the literature, although such discussions have emphasized carrier recombination processes important for light emitting diodes (LEDs) or light displays. One published paper (Characteristics and Potential Applications of GaAs.sub.1-x P.sub.x MIS Structures, Solid-State Electronics, 1974, Vol. 17, pp 25-29) discloses a GaAsP MOS structure for producing surface electroluminescence and hence a light display. This work points out a characteristic of GaAsP, viz, long charge-holding capability and low thermal generation of minority carriers. This paper, while addressing the matter of "imaging applications", clearly intends `image-producing` as opposed to `image-sensing`. In the present invention, `image sensing` is addressed for the first time in GaAsP devices based upon MOS structures. As will be more fully discussed below, the present invention employs a GaAsP Mos structure: The MOS GaAsP structure has a high concentration of interface traps, a characteristic which has discouraged the use of GaAsP for light-sensing purposes. Such interface states trap light-produced minority carriers and seriously limit the transfer efficiency of GaAsP in charge-coupled device (CCD) applications. However, the present device employs a charge injection technique to sense light levels.
The published paper (Noise Linearity and Trapped Charge Measurements with Charge Sensitive Amplifiers, published in the Proceedings of the 1975 International Conference on the Application of CCD, October 29-31, San Diego, Calif.) discusses the measurement of trapped charges in CCDs by the use of a charge-sensitive amplifier. And, in charge-injection devices (CIDs), as disclosed in U.S. Pat. No. 3,805,062, use has been made of the difference in capacitance between a pre- and post-charge injection, as a measure of light-produced minority charges.